I have an MSI R9 280X and i love it so far. My target is to achieve 30-35 FPS in Crysis 3 on Ultra(Very High) in 4K. If i buy another R9 280X from ebay, i can easily achieve this, but so can a GTX 980 whose crazy low power consumption i really like but still doesnt mathematically make up for the 250 dollar difference (sell the 280X for 150 and buy the 980 for 550).
I probably would aim for an GTX 980 cause Dual GPU's setup's are going to give you nothing but headaches especially on the AMD Side of things, but from what i'm aware Crossfire is improved in the Hawaii Cards, not the Rebranded 7000 series cards. and i would grab GTX 980, for two reason's the 900 series cards are monsterous overclockers according to multiple sources and performance wise it performs above a 780ti for $550 thats a really good deal. but if you are willing to spend around 600 bucks to upgrade your setup, 2 GTX 970's are also something you can consider for 4K
Screen-Tearing much more often (this is why solutions like G-Sync and Project Free-Sync [AMD's Equivalent] exist. to remove Screen-Tearing COMPLETELY)
Potential Graphical Glitches - perfect example is Battlefield 4, They game was next to broken with Dual GPU Setups, (this later on got fixed but depending if you play old games much more often you will see graphical glitches.
Over time, Dual GPU System's will get more prominent when we have no more screen tearing. cause it will get point where people are going to want a smoother experience (whether they use G-SYNC or AMD's Free-Sync) Overall though a GTX 980 will do 4k but i will say barely, (depends on the Game) Crysis at 4K, that game SHITs on GPU's it always has. you're going to have to wait about another generation or so for 4K to be do-able on a Single GPU card. but if i were you and I WANTED 4K right now i would grab 2 GTX 970's. you could probably get away with that. at least for now, AMD hasn't done anything to combat the GTX 970 or GTX 980 yet, i would wait a bit to see who gives you the better performance for the money. according to benchmarks and follow through with that, right now the GTX 970 and GTX 980 beats the Hawaii GPU's in most games with the exception of the AMD Optimized Titles, but like i mentioned i would wait a bit to see how AMD answers Nvidia's Maxwell. cause I believe AMD will Raise Hell to Combat Nvidia's Maxwell. whether they drop the R9-290 and 290x to an ABSURDLY low price or Drop the R9-295 and R9-295x to replace the R9-290 and R9-290x
Most probably, with 2 * R9 290X OC, i will get around 33 fps in crysis3. With 2 * GTX 970 maybe i will get ~40. Honestly, 360$ extra for that little benefit seems like a little too much. I will just get a second R9 280X and wait for a more 4k capable generation of GPUs.
Or maybe i should wait for the 970s to hit the used market...